The present invention relates generally to a system and method for reducing the transmission of spurious radiation from an antenna. More specifically, the present invention relates to a system and method for spreading out the transmission of spurious radiation produced by local oscillators in a spacecraft-based phased array antenna.
A typical problem of spacecraft-based transmitting antennas is the radiation of spurious out-of-band transmissions. Satellite antenna spurious emission specifications require that the power level of the spurious out-of-band transmissions received on the Earth be less than a maximum power level of typically around xe2x88x9260 dBc relative to the received communication signal over a 4 kHz band. Often, spurious out-of-band emissions produced by transmitting antennas will be spread out over the entire frequency band and therefore it is not difficult to meet this specification requirement. However, local oscillators (LOs) used aboard spacecraft based phased array antennas produce an LO signal having a single frequency tone. A portion of this LO signal typically leaks into the communication signal radiated from the antenna. Because the LO signals are at a single frequency tone, the energy from these LO signals are concentrated within much less than a 4 kHz band. This makes it difficult to meet the spurious emission specification requirement because all the energy from the spurious signal is concentrated within a 4 kHz band. What is needed is an effective method of reducing the level of spurious transmissions transmitted to the destination coverage area that is not too costly or overly complex to implement.
The present invention is a system and method for reducing the transmission of spurious radiation produced by local oscillators in a spacecraft-based phased array antenna. This spurious radiation is produced by leakage from local oscillator signals in the antenna system. A portion of the local oscillator (LO) signal leaks into the output signal produced by an upconverter. That LO signal leakage is then transmitted from the antenna radiating element as a spurious signal at LO frequency. Since all the LO signals in each elemental path of a conventional IF beamformed phased array have approximately the same phase, the LO signals are all at nearly the same phase when radiated. This causes a strong LO leakage signal transmitted in the boresight direction of the antenna.
The system of the present invention reduces the amount of spurious radiation received at the coverage area of the antenna on the Earth by spreading a substantial portion of the transmitted LO leakage signal outside of the earth disk. Alternatively, a substantial portion of the transmitted LO leakage signal can be spread outside of the desired coverage area. From the perspective of a geostationary satellite, the earth disk is 17.6 degrees in diameter. The antenna thus spreads most of the radiated LO signal power beyond this 17.6 degree disk.
One method of spreading the LO radiated signal outside of the coverage area is to adjust the phase of the LO signal to a specific value in each elemental path. One configuration is to shift the phase of the LO signal in every other elemental path by 180 degrees (or 180xc2x1360 n degrees, where n is any integer). This will cause the LO radiation pattern to have a null in the direction of the earth center (assuming that the antenna is transmitting towards the earth), with 4 main lobes separated by 90 degrees in phi-angle around the earth circle and with their peak at approximately 2 times the maximum edge-of-coverage theta-angle (this radiation pattern is specific to a phased array antenna in a geostationary orbit having 2xcex spacing between radiating elements, where xcex is the wavelength of the LO signal).
One method of accomplishing the phase shift of the LO signal is to insert a transmission line having a length of xcex/2 in the LO signal path prior to entering the mixer in every other elemental path, where xcex is the wavelength. of the LO signal. This will introduce a phase shift of 180 degrees into the LO signal in every other elemental path. A phase shifter or transmission line in the IF elemental path introduces an offsetting phase shift of xe2x88x92180 degrees, so that the phase of the RF communication signal transmitted by the antenna is not affected by the introduction of the transmission line into the LO signal path.
One embodiment of the transmitting antenna of the present invention includes L beamformers each having N elemental paths, where L and N are positive integers. The antenna also includes N upconverters, one for each elemental path. Each upconverter has a first input coupled to an output of a corresponding elemental path. Each upconverter has a second input receiving a local oscillator signal. N radiating elements are each coupled to an output of a upconverter in the corresponding elemental path. A phase adjustment device, such as a length of transmission line, is coupled to at least one of the upconverters at the second input, wherein each phase adjustment device adjusts the phase of the oscillator signal provided to the corresponding upconverter so as to substantially spread a leakage transmission of the oscillator signal outside the coverage area. In one configuration, the N elements are located in a two dimensional grid. The phase adjustment devices are located in every other elemental path and introduce a phase shift of +180 degrees in the local oscillator signal in that elemental path.